A conventional gas turbine engine includes various rotor blades in the fan, compressor, and turbine sections thereof which are removably mounted to respective rotor disks. Each of the rotor blades includes a retention dovetail at the radially inner end thereof which may either be an axial-entry dovetail or a circumferential-entry dovetail. In axial-entry dovetails, the rotor disk includes a plurality of circumferentially spaced apart, axially extending dovetail slots for slidably receiving the blade dovetails for retention therein. And, for the circumferential-entry dovetails, the rotor disk includes a single circumferentially extending dovetail slot which circumferentially slidably receives the complementary dovetails for retention therein. In the axial-entry rotor disk the dovetail slots define a plurality of circumferentially spaced apart dovetail posts which carry the centrifugal loads from the blades; and in the circumferential-entry rotor disk only two axially spaced apart and annular dovetail posts are defined by the circumferentially extending single dovetail slot therebetween.
In view of the structural differences in axial dovetails and circumferential dovetails, the corresponding rotor disks are designed differently. Axial dovetails in one of their simplest configurations include a pair of lobes in a symmetrical dovetail configuration which is straight in the axial direction and configured for retention in a complementary dovetail slot in the rotor disk which is disposed parallel to the axial centerline or rotation axis of the rotor disk without slope in a vertical plane extending radially through the axial centerline axis, and parallel to the centerline axis without skew in a top or plan view looking along the circumference of the rotor disk.
In another conventional configuration, the dovetail slots in the rotor disk may be skewed or inclined relative to the centerline axis of the rotor disk in the top, plan view which is referred to as skew, while also being parallel to the centerline axis in the vertical view without slope. The dovetail slot is again straight, and the blade dovetail is similarly straight and configured for retention therein.
And, in yet another configuration, the dovetail slots in the rotor disk are both skewed and sloped, with inclination thereof both in the plan view along the circumference of the rotor disk, i.e. skew, and in the vertical sectional view, i.e. slope, relative to the centerline axis. The corresponding blade dovetail is again straight and configured for retention in the skewed and sloped dovetail slots. This configuration is primarily used in gas turbine engine compressors at the stage-one position thereof with a relatively high axial slope of the outer rim of the rotor disk for improved aerodynamic performance. The blade dovetails typically have corresponding slope in order to be axially retained therein without excess weight. And dovetail skew is provided in order to better align the twisted airfoil with its dovetail to reduce the stresses therein due to centrifugal force of the blades during operation.
More specifically, a typical rotor disk includes an outer rim which contains the dovetail slots for retaining the rotor blades thereto, with an integral and thinner annular web extending radially inwardly therefrom, followed in turn radially inwardly by an axially thicker hub. This provides a relatively low weight and structurally efficient rotor disk effective for carrying the substantial centrifugally generated loads from the blades within acceptable stress limits for providing a useful life of the disk during operation. Axially sloping the disk rim provides a smaller circumference at the forward end of the rim which has a smaller diameter, with a relatively larger circumference at the aft end of the rim which has a larger diameter. In high solidity compressor blade configurations, the number of compressor blades on the disk is made as large as possible for aerodynamic reasons. However, since the forward end of the blade rim has a smaller circumference than the aft end thereof, the blades are spaced closer together at the forward end than at the aft end, with the dovetail posts in the blade rim defined by the dovetail slots being circumferentially thinner at the rim forward end than at the rim aft end. The centrifugal loads generated by the blades during operation therefore create higher reaction stresses in the dovetail posts at the forward end thereof than at the aft end thereof.
Furthermore, since typical blade dovetails are straight for allowing economical fabrication of the corresponding dovetail slots by using linearly translated manufacturing cutting broaches, such broaches when used to form the skewed dovetail slots in the disk rim necessarily vary the radial configuration of the dovetail posts. This is better appreciated by recognizing that a straight dovetail slot extending axially through a disk rim without either slope or skew results in a constant configuration dovetail post. However, by skewing the dovetail slot it necessarily extends also circumferentially around the circumference or curvature of the rim which therefore varies the configuration of the corresponding dovetail posts. With the addition of slope to the dovetail slot, the configuration of the resulting dovetail posts is yet further affected.
Accordingly, in a skewed-only or skewed and sloped dovetail slot configuration, the resulting reaction stresses in the dovetail posts becomes a more complex design problem which must be resolved in order to obtain acceptable levels of centrifugally induced stresses with a suitable useful life of the rotor disk.
For example, in a rotor disk design without slope or skew the reaction forces carried through each dovetail post from the corresponding blade dovetails are symmetrical and intersect each other along the radial centerline axis of the dovetail posts and therefore create primarily tensile stresses in the neck portion of the dovetail post without bending stresses therein.
However, in the skewed design without slope, only the axial center section of the rotor disk experiences no bending of the disk post necks. Both axially forwardly and axially rearwardly from the center section, the angles of inclination of the resultant reaction forces acting on the opposing lobes of each dovetail post are no longer symmetrical but intersect each other to either circumferential side of the radial centerline axis of the dovetail post thus effecting a bending moment which induces bending stress in the dovetail post neck. However, the direction of the reaction bending moment has one sense axially forward from the center of the disk, and an opposite or negative sense relative thereto in the axially rearward direction from the center of the disk rim which effectively balance each other out with substantially equal maximum values of bending stress in the respective portions of the disk post neck.
In the skewed and sloped configuration, the resultant reaction loads carried by the opposing lobes of each dovetail post are again not symmetrical and therefore induce bending stresses in the dovetail post necks, and are not symmetrical without bending at the center of the disk rim as in the skewed-only configuration which therefore results in an unbalanced configuration with a maximum bending stress occurring in the dovetail posts adjacent the forward end of the disk rim having the minimum diameter, with reduced bending stresses occurring at the aft end of the disk rim having the largest diameter. Since the forward, smaller diameter end of the disk rim as compared to the aft, larger diameter end of the disk rim has less material for carrying the centrifugal loads, the stresses thereat are increased which decreases the useful life of the rotor disk.